How do I encourage new Holly branches?

I had to cut the holly trees in my front yard way, way back last year. Unfortunately, they had been growing from some large branches near the ground on one side only. Now that I've cut them way back, the new growth is only from one side. Is there a way I can nick the outer bark on the other side to encourage growth of new branches from other areas where I want growth so as to end up with an evenly growing tree?

Not sure if it's even possible, but I have grown new branches and roots in rubber trees, for example, by cutting away the bark in a certain area, then applying root compound to encourage new roots in that specific area. Can I do something similar with holly trees?

I don't know if anything will work. I'd be reluctant to inflict damage on the bark to a plant growing outdoors, because that might allow any number of nasty things to gain access to the plant's living tissues.

Is there enough space between the hollies that you could interplant with new young plants to fill in?